The rapid increase in biotechnology along with the consumer's desire for tastier, healthier and more aesthetically pleasing foodstuffs and beverages has contributed to a need for highly selective, efficient and cost-effective processes for separating various chemicals from complex mixtures. While highly selective chromatographic adsorbent and membrane separation processes have gained wide acceptance among industrial users, the demand for better separation materials is rapidly moving beyond the realm of conventional adsorbents.
With respect to adsorbent resins, basically two types of styrenic adsorbents are commercially available, i.e., porogen modified styrene/divinylbenzene adsorbents and methylene-bridged styrene/divinylbenzene adsorbents.
In the diluent or porogen-modified styrene/divinylbenzene adsorbents, the amount of divinylbenzene ranges from 35 to 80 weight percent in order to impart pore stabilization and rigidity to the polymer structure and to prevent pore collapse. The porogens, such as toluene and/or iso-octane, are chosen to control the pore size distributions. The closer the solubility parameter of the porogen to the solubility parameter of the polymer, the more is mesoporosity, i.e., pores with diameters of from 2 to 20 nanometers (nm), preferred. Similarly, macroporosity (&gt;20 nm) is favored when the solubility parameters are very different and/or the porogen volumes are high. In either case, the amount of microporosity (&lt;2 nm) is usually small, ranging up to about 0.15 cc/g. Consequently, the surface area for such adsorbents typically range from about 400 to about 900 square meters per gram (m.sup.2 /g) depending upon the amount of crosslinker.
Furthermore, according to these methods, the pores in the polymer are formed during the polymerization. Therefore, if an attempt is made to substantially develop the pores, the resin strength tends to decrease substantially. Accordingly, the proportion of pores is necessarily limited. Thus, it has been difficult to produce a product having a high porosity.
In order to increase porosity and surface area, such highly crosslinked copolymer resins have been treated with Lewis-acid catalysts in the presence of a non-swelling liquid (U.S. Pat. No. 5,218,004) and in the presence of a swelling solvent (U.S. Pat. No. 4,543,365). However, in neither case is the highly crosslinked copolymer resin haloalkylated and bridged; rather, the porous structure is believed to be modified either by the reaction of pendant vinyl groups or by the reorganization of existing crosslinkages by breakage and reformation.
The methylene-bridged, styrene/divinylbenzene adsorbents, on the other hand, are produced from lightly crosslinked gel or macroporous copolymers that possess good swelling characteristics (U.S. Pat. No. 4,263,407 and U.S. Pat. No. 4,950,332). Thus, the amount of crosslinking agent in such polymers is typically less than 8 percent. The gel phase of the copolymers undergoes considerable expansion during the bridging process, producing large amounts of microporosity ranging up to about 0.7 cc/g. The methylene bridging serves to lock the polymer structure in place while swollen and to prevent pore collapse. If macroporous copolymers are used, the adsorbents can exhibit considerable macroporosity as well; total surface area can range up to about 1600 m.sup.2 /g. Unfortunately, mesoporosity remains on the low side, i.e., less than 0.5 cc/g.
In chromatographic separations, it is well established that high surface area, i.e., a high level of microporosity, increases the adsorption capacity of adsorbents for small molecules, while a high level of macroporosity and mesoporosity improves adsorption kinetics. Thus, porogen-modified, styrene/divinylbenzene adsorbents, typically characterized as having mesoporosity greater than 0.5 cc/g and microporosity less than 0.15 cc/g, generally exhibit excellent exchange kinetics but poor adsorption capacity. Conversely, methylene-bridged, styrene/divinylbenzene adsorbents, typically characterized as having mesoporosity less than 0.5 cc/g and microporosity greater than 0.2 cc/g, tend to have sufficient adsorption capacity but slow exchange kinetics. It would be desirable to combine the advantages of high capacity and rapid kinetics in a single adsorbent.